The present invention relates generally to a light beam phase modulator, and more particularly to a thermal modulator for use with an optical fiber interferometer.
Modulators for changing the phase of a coherent light beam passing through an optical waveguide such as an optical fiber are well-known to those skilled in the optical arts. Such modulators can be found in interferometers of the type wherein a physical parameter is sensed by directing light through an optical waveguide which is adapted to be altered by the physical parameter, and measuring the change in the phase of the light caused by the physical parameter by measuring the interference pattern created by combining the light with light from a reference optical waveguide. Examples of such interferometers include Mach-Zehnder and Michelson interferometers. One type of modulator used in interferometers is an electrically resistive element disposed on one of the optical waveguides. Current is supplied to the element for the purpose of heating the waveguide on which it is disposed (via I.sup.2 R losses), thereby proportionately varying the waveguide's length and refractive index and hence its optical path length. This change in optical path length shifts the phase of the light beam passing through the waveguide, which shift can be measured by measuring the resulting change in the interferometer's interference pattern. U.S. Pat. No. 5,047,626 to Bobb et al. discloses the use of such an electrically resistive element on one of the optical waveguides of a Mach-Zehnder interferometer as the sensing element of the interferometer. Physical parameters which can be measured by measuring heat loss, such as a fluid's thermal conductivity, are measured by heating the resistive element and then measuring the change in temperature of the fiber as it cools by monitoring the change in interference pattern which occurs, the rate of change of the temperature being influenced by and indicative of the parameter being measured.
Such Mach-Zehnder interferometers often include means for maintaining quadrature between the two light beams by controlling the optical path length of one of the optical waveguides. U.S. Pat. No. 4,859,059 to Bobb et al. discloses maintaining quadrature by supplying current to an electrically resistive modulator disposed on one of the optical waveguides.
Heating the resistive element with current in these applications involves running electrical leads thereto. This can be dangerous when sensing physical parameters of a flammable liquid. In other applications, the sensing environment may have a high potential or electric field (such as when microwaves are being used), in which case electrical leads would be undesirable because they would provide a conductive path to ground. For other applications, the sensing environment may not provide sufficient space for electrical leads, as when the sensing environment is a small amount of fluid in a confined cell or when the sensor is a probe for the human body.